Enhanced production and efficient utilization of lignocellulosic products are issues of high importance to both the pulp and paper industry and society. The production of mechanical and chemimechanical pulps is an efficient way of using the world's natural resources since the yield of these manufacturing processes is high and the environmental impact is relatively low. Mechanical and chemimechanical pulping constitute about 25% of the total virgin fibre production in the world. One drawback with mechanical pulping processes is the high energy consumption that represents about 20% of the energy demand of papermaking in the world. The energy alone represents 25-50% of the total manufacturing cost of a thermomechanical pulp (TMP) depending on where in the world the mechanical pulp mill is located. In a TMP mill, about 80% of the energy is consumed during mainline refining (primary, secondary etc.), reject and low-consistency refining. The rest of the energy is consumed in pumps, agitators, screens, blowers, fans and mechanical drives. This means that most of the energy is used for fibre separation and for developing the fibres to make them suitable for the defined end-usage. It is therefore extremely important to find suitable ways of reducing the consumption of energy. However, a process that reduces the energy consumption during production of mechanical pulp is of limited interest for conventional products if the pulp or paper strength is, at the same time, substantially reduced or if the environmental effect is substantially impaired.
EP 494 519 A1 relates to a process comprising impregnating chips with an alkaline peroxide solution containing stabilizers for peroxide followed by mechanical defibration, in which the wood chips are pre-treated prior to peroxide impregnation. However, the process of EP 494 519 A1 involves extensive capital investment and does not result in sufficient energy saving with maintained pulp yield and pulp properties.
One object of the invention is to reduce the energy consumption in a process which is simple to install in a high-yield pulping process and without substantially reducing the fibre length or strength properties of the produced pulp. A further object of the present invention is to provide such a process while maintaining the pulp yield at an acceptable level. A further intention of the present invention is to provide a facilitated process without need of considerable capital investments. A further intention is to provide a process in the absence of alkaline treatment stages while improving or at least not substantially affecting properties of the obtained high-yield pulp, e.g. strength properties.